1. Field of the Invention
The present invention relates to a gene which determines cytoplasmic sterility in plants, such as of the Cruciferae family, as well as a method for producing hybrid plants by utilization of this gene. Specifically, the present invention relates to a gene for cytoplasmic male sterility and a method for producing hybrid plants by utilizing this gene for the purpose of developing new F.sub.1 plant varieties.
2. Description of the Related Art
First filial generations are utilized for many major crop and vegetable plants. First filial generations are characterized by; 1) superior agricultural characteristics resulting from hybrid vigor, 2) homogeneity of harvested products, 3) segregation of genetic characteristics in the subsequent generations, thus protecting the profits of the breeder.
In the Cruciferae family, self-incompatibility is widely used for harvesting F.sub.1 seeds. However, in rapeseed, which does not have a stable self-incompatibility system, there is a demand for systems to generate F.sub.1 seeds using cytoplasmic male sterility instead (hereafter abbreviated as "cms"). At present, techniques for generating F.sub.1 seeds possessing polima cms are being applied. However, because its male sterility characteristic is unstable and flower morphology is poor, which affect the yield of harvest, there is a need for improvement.
Recently, attempts have been made to use Ogura cms derived from Japanese radish in rapeseed in place of polima cms. Male sterility in Ogura cms is stable, and fertility can be restored by a single fertility-restoring gene (hereafter abbreviated as "Rf gene"). The Rf gene of Ogura radish has already been introduced from Japanese radish into rapeseed and it has been ascertained that both the cms and Rf genes of Ogura radish introduced in rapeseed can be put into practical use without problems.
However, cms cytoplasm not only causes pollen sterility, it may also affect other characters of plants. Formerly, one type of maize cytoplasm, T-cms, had been widely used in the generation of F.sub.1 seeds, but it conferred susceptibility to two major plant diseases, sesame leaf bright and yellow leaf bright diseases, as well as susceptibility to damage by Ostrinia nubilalis. In the 1970's, the first filial corn suffered a severe damage due to major outbreaks of sesame leaf bright. Since then it has been recognized that the use of only a single type of cytoplasm for the production of F.sub.1 seeds is very dangerous.
In addition, cms cytoplasm may influence flower morphology. For example, in the case of rapeseed carrying polima cms cytoplasm it is known that large openings may form in the base of the petal through which bees are able to obtain nectar. This circumvents the normal process of pollen transfer, ultimately leading to problems in the yield of seed in polima cms plants. In addition, flower size in Japanese radish carrying Ogura cms cytoplasm is smaller than that in fertile individuals and the volume of secreted nectar is low, which results in infrequent visits by pollinating insects. Thus, Japanese radish carrying Ogura cms cytoplasm also exhibit problems in seed yield. When Japanese radish carrying cms cytoplasm derived from Kosena radish ("Kosena") was tested, it was found that the seed yield was higher than that in plants in which Ogura cms cytoplasm had been used. This suggests that the cms cytoplasm from Kosena is genetically different from that of Ogura. That is, this phenotypic difference appears to be caused by the result of the interaction between cytoplasmic genetic factors, viz. mitochondria and chloroplasts, and nuclear genes. It was thought that by introducing this cytoplasm by crossing or cell fusion into useful crops other than Japanese radish there would be a high probability of obtaining varieties with superior morphological characteristics.
In addition, the negative effects resulting from incompatibility between the nucleus and cytoplasm, which occurs when cms cytoplasm is introduced into heterologous plant species, can be removed to some degree by cell fusion. The cytoplasmic genomes (chloroplasts and mitochondria) in cytoplasmic hybrid plants (cybrids) formed by cell fusion often consist of recombinants derived from both parental genomes. Using this phenomenon, we could select cybrids into which only the cms gene has been introduced. In this process, It was thought that the introduction of cms cytoplasm which does not cause problems with respect to petal or nectar yield would increase chances of obtaining cybrids advantageous for plant breeding.